1. Field of the Invention
The invention relates to a gas-operated premixing burner for the combustion chamber of, for example, a gas turbine in which, within a premixing space, the fuel injected by means of a plurality of nozzles is intensively mixed with the combustion air prior to ignition, the nozzles being arranged around a burner axis.
2. Discussion of Background
Gas turbine combustion chambers based on premixing burners are known, for example from U.S. Pat. No. 4,408,461 to Bruhwiler et al. In such a burner, a premixing/pre-evaporation process takes place at a large excess air ratio between the injected fuel and the compressor air within a number of tubular elements before the actual combustion process takes place downstream of a flame holder. The pollutant emission figures from the combustion can be substantially reduced by this measure. Combustion with the largest possible excess air ratio--assuming that the flame still burns at all and also that not too much CO is produced--then reduces not only the NO.sub.x pollutant quantity but, in addition, also keeps the level of other pollutants low, namely the CO already mentioned and unburnt hydrocarbons. This permits the selection of a larger excess air ratio, and although larger quantities of CO are produced initially, these can react further to form CO.sub.2 so that finally, the CO emissions remain small. On the other hand, however, only a small amount of additional NO is formed because of the large amount of excess air. Because a plurality of tubular elements undertake the premixing in this known combustion chamber, the number of elements operated with fuel during load control is just that number which provides the optimum excess air ratio for the particular operating phase (start, part load, full load).
Other types of premixing burners in which it is possible to dispense with the flame holder are known, in the form of double-cone burners, from for example, U.S. Pat. No. 4,932,861 to Keller et al.
All of the combustion chambers with premixing burners, however, have the shortcoming that the flame stability limit is reached, at least in the operating conditions in which only a part of the burners are operated with fuel or in which a smaller quantity of fuel is admitted to the individual burners. In fact, the flame-out limit has already been reached at an excess air ratio of approximately 2.0 under typical gas turbine conditions because of the very weak mixture and the resulting low flame temperature.
This fact leads to a relatively complicated method of operating the combustion chamber with correspondingly complex control. Supporting the burner by means of a small diffusion flame is regarded as another possibility of extending the operating range of premixing burners. This pilot flame receives pure fuel or at least badly premixed fuel. Although this leads to a stable flame, it does involve the high NO.sub.x emissions typical of diffusion combustion.